Method and system for the removal and/or avoidance of contamination in charged particle beam systems

ABSTRACT

A charged particle beam system is disclosed, comprising:
         a charged particle beam generator for generating a beam of charged particles;   a charged particle optical column arranged in a vacuum chamber, wherein the charged particle optical column is arranged for projecting the beam of charged particles onto a target, and wherein the charged particle optical column comprises a charged particle optical element for influencing the beam of charged particles;   a source for providing a cleaning agent;   a conduit connected to the source and arranged for introducing the cleaning agent towards the charged particle optical element;
 
wherein the charged particle optical element comprises:
   a charged particle transmitting aperture for transmitting and/or influencing the beam of charged particles, and   at least one vent hole for providing a flow path between a first side and a second side of the charged particle optical element,
 
wherein the vent hole has a cross section which is larger than a cross section of the charged particle transmitting aperture.
       

     Further, a method for preventing or removing contamination in the charged particle transmitting apertures is disclosed, comprising the step of introducing the cleaning agent while the beam generator is active.

TECHNICAL FIELD

The invention relates to methods and systems for cleaning surfaces incharged particle beam systems, and/or at least partially avoidingcontamination of the surfaces. Such charged particle beam systems maycomprise charged particle lithography systems or electron microscopes,for example.

BACKGROUND

In charged particle beam systems a target surface may be exposed to oneor more charged particle beams directed to and focused on the surfacewith high accuracy.

In charged particle beam lithography systems, small structures may beformed with high accuracy and reliability. In charged particle multiplebeam lithography the pattern formed on the surface is determined by theposition where each individual beam interacts with the resist on thesurface. In other charged particle beam exposure systems, such aselectron microscopes, samples may be analyzed based on the interactionof the charged particles with the sample. Therefore, compliance of thebeams reaching the surface with specified beam properties, such as beamposition and intensity, is of high importance.

The accuracy and reliability of charged particle beam systems isnegatively influenced by contamination. Charged particle beam systemscomprise charged particle optical elements for projecting one or morebeams of charged particles onto the target surface. An importantcontribution to contamination in charged particle beam systems isaccumulation of deposits of contaminants on surfaces, such as surfacesof the charged particle optical components.

In charged particle beam systems, such as electron microscopes, e.g.scanning electron microscopes, and charge particle lithography systems,the charged particles may interact with residual gases, or contaminants,e.g. hydrocarbons, present in the system. Such contaminants may arisefrom outgassing from components within the system and/or from the targetto be exposed. The interaction between charged particle beams andcontaminants may cause Electron Beam Induced Deposition (EBID) or IonBeam Induced Deposition (IBID) on surfaces of the charged particleoptical elements. Contamination layers, formed by EBID or IBID, mayperturb the functioning of these elements, and hence negativelyinfluence projection of charged particles on the target surface. Removalof contamination, or prevention of contamination growth, in particularin areas with relatively high hydrocarbon partial pressures andrelatively high beam current densities, is therefore highly desirable.

A method for removing contamination is described in US 2015/028223 A1,also by the applicant. US 2015/028223 A1 describes an arrangement and amethod for transporting radicals, for example for removal of contaminantdeposits. The arrangement includes a plasma generator and a guidingbody. The plasma generator includes a chamber in which plasma may beformed. The chamber has an inlet for receiving an input gas, and one ormore outlets for removal of plasma and/or radicals created therein. Theguiding body is arranged for guiding radicals formed in the plasmatowards an area or volume at which contaminant deposition is to beremoved. Further a charged particle lithography system comprising sucharrangement is described.

Although the method and arrangement described in the document citedabove enable cleaning within charged particle lithography systems, theefficiency of the cleaning, in particular the rate of removal ofdeposits from surfaces, is observed to be limited. It is an object ofthe present invention to provide a method and a system which reduce thecontamination in charged particle beam systems and/or increase thecleaning efficiency in charged particle beam systems.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a chargedparticle beam system, comprising:

-   -   a charged particle beam generator for generating a beam of        charged particles;    -   a charged particle optical column arranged in a vacuum chamber,        wherein the charged particle optical column is arranged for        projecting the beam of charged particles onto a target, and        wherein the charged particle optical column comprises a charged        particle optical element for influencing the beam of charged        particles;    -   a source for providing a cleaning agent;    -   a conduit connected to the source and arranged for introducing        the cleaning agent towards the charged particle optical element;        wherein the charged particle optical element comprises:    -   a charged particle transmitting aperture for transmitting and/or        influencing the beam of charged particles, and    -   a vent hole for providing a flow path between a first side and a        second side of the charged particle optical element,        wherein the vent hole has a cross section which is larger than a        cross section of the charged particle transmitting aperture.

The vent hole has a conductance which enables a flow of contaminantspecies through the charged particle optical element. A flow from oneside of the charged particle optical element provides a reduction inpressure at the charged particle optical element, compared to thesituation without a vent hole. Thereby the amount of species availablefor contamination growth due to EBID or IBID at or in the chargedparticle transmitting aperture is reduced. The cross section of the venthole and/or the number of vent holes provided is generally chosen toenable sufficient pressure reduction. The vent hole is provided inaddition to the charged particle optical transmitting aperture, which isoften of a relatively small dimension. The cleaning agent facilitatesremoval of contamination from surfaces of the charged particle opticalelement. Hence, the system according to the first aspect not onlyenables efficient cleaning within charged particle beam systems, butalso prevents or at least reduces the probability of formation and/orgrowth of contamination layers on the charged particle optical element.

Vent holes are particularly advantageous in regions of the chargedparticle optical column having a limited conductance from an interior ofthe column to one or more vacuum pumps connected to the chamber. In suchregions there may be a non-negligible pressure near or at the chargedparticle optical elements. Vent holes are also advantageously providedin systems where the distance between the charged particle opticalcolumn and the target is very small, which restricts the flow path fromthe portion of the target surface located below the column to the vacuumpump. In such systems, species degassing or desorbing from the targetsurface may, at least to some extent, enter the charged particle opticalcolumn. Examples of such systems are multi-beam systems for projecting amassive amount of charged particle beams onto the target surface. Inmulti-beam systems the space between the final element of a projectionlens and the target surface is often so small that molecules or clustersof the resist layer, evaporating or otherwise leaving a portion oftarget surface located below the projection lens experience a restrictedor limited flow path toward the vacuum pump. As a result these speciesmay to a non-negligible extent enter the charged particle optical columnthrough projection lens apertures. This leads to a presence of species,such as C_(x)H_(y) compounds, close to a charged particle opticalelement arranged upstream the projection lens, such as a beam stopelement. The species can cause to contamination of this charged particleoptical element, in particular the charged particle transmittingapertures thereof.

It can be mentioned that a flow, e.g. of contaminant species, is notnecessarily limited to pass through the vent hole. Species may flow froman interior of the charged particle optical column toward the vacuumpump via a vent hole, but may also, at least to some extent, exit thecharged particle optical column via paths not passing through a venthole. Species leaving the target surface may, at least to some degree,flow toward the vacuum pump via the space between the target surface andthe charged particle optical column, without entering the chargedparticle optical column.

In some charged particle optical elements, the charged particletransmitting aperture is of a diameter such that the beam of chargedparticles passes through the aperture at close distance to the edgesthereof or even at least to some part impinges on the charged particleoptical element. Such charged particle optical element is sensitive tocontamination, due to the risk of deposited contamination reducing thesize and/or changing the shape of the aperture, or even clogging theaperture. A reduction in aperture size may lead to loss of transmissionthrough the aperture, and a change in aperture shape may lead to achange in cross section of the charged particle beam and/or a change inthe way the charged particle optical element influences the chargedparticle beam. Contamination at the aperture may be subject to charging,which may perturb the trajectory of the charged particle beam.

The charged particle beam system may be any type of charged particleexposure system, for example a charged particle multi beam lithographysystem, or an inspection system, such as any type of electron microscopeor a tool using ions for analyzing a sample. The charged particles maybe electrons or any kind of ions used in the above systems.

The charged particle optical element may also be referred to as electronoptical or ion optical element or lens. Influencing the beam of chargedparticles comprises one or more of changing the energy of the chargedparticles, deflecting a charged particle beam, thereby changing thedirection of the beam, stopping or at least partly blocking the beam,for example acting as a current limiting aperture or forming a pluralityof charged particle beams from a beam of charged particles, focusing,defocusing or diverging charged particle beams, etc. The conduit isarranged to introduce, guide, or direct the cleaning agent toward, onto,or over the charged particle optical element, or a surface thereof,comprising one or more charged particle transmitting apertures, therebyenabling cleaning thereof.

In an embodiment, the source for providing a cleaning agent is a sourceas described in US 2015/028223 A1. Alternatively another type of plasmasource, a molecular gas source, or a generator of active species, forexample an ozone generator, can be used.

In an embodiment, the cleaning agent comprises atomic oxygen radicals,molecular oxygen gas, molecular or atomic oxygen ions, and/or ozone.Alternatively, other types of species or molecules can be used. Goodresults have been observed using a mixture of atomic oxygen radicals andmolecular oxygen. The inventors have observed that such mixture, inparticular in presence of charged particle beams, enables efficientremoval of contamination without disturbing the functioning of thecharged particle beam system. Preferably the source is configured toprovide a controlled flow of cleaning agent.

In some systems, such as charged particle lithography systems, thetarget surface may deteriorate upon contact with the cleaning agent.Therefore, the flow of cleaning agent and the total vent hole crosssection are typically determined from a trade-off between providingefficient cleaning of charged particle optical elements by the cleaningagent and providing sufficient pressure reduction at the chargedparticle optical element, while avoiding flow of cleaning agent to thetarget surface.

In an embodiment, the charged particle optical element comprises aplurality of vent holes and a plurality of charged particle transmittingapertures, the vent holes being arranged next to the charged particletransmitting apertures. This enables prevention of contamination growthin a multi-beam system. The charged particle transmitting apertures maybe arranged in one or more groups, or arrays. The vent holes arepreferably arranged adjacent and/or between such groups of chargedparticle transmitting apertures. An area of the charged particle opticalelement comprising one or more groups of charged particle transmittingapertures is often referred to as beam area, as it represents an areawithin the trajectory of charged particle beams through the chargedparticle optical column. Analogously, an area which is not intended forreceiving charged particle beams, and which is located outside a chargedparticle beam trajectory is referred to as non-beam area. Chargedparticle multi-beam systems often comprise a plurality of elongated beamareas and non-beam areas arranged in an alternating, periodic manner,each beam area positioned between two non-beam areas. Such arrangementis described in U.S. Pat. No. 8,653,485 and U.S. Pat. No. 8,492,731 ofthe applicant. The vent holes are preferably arranged in one or morenon-beam areas. In some embodiments, the vent holes are arrangedimmediately adjacent the one or more beam areas. Alternatively and/oradditionally, vent holes can be provided in beam areas. In the lattercase, however, the risk of having charged particles pass through a venthole is higher.

In an embodiment the charged particle optical element comprises asubstantially flat substrate, wherein the vent hole is provided by athrough-hole extending through the substrate. The through-hole ispreferably oriented substantially straight through the substrate. In anembodiment the substrate comprises a silicon substrate provided with acoating, for example a coating comprising molybdenum.

In an embodiment, the vent hole has a circular cross section. Thecharged particle transmitting apertures are generally also circular. Thediameter of the vent hole is then larger than the diameter of thecharged particle transmitting apertures.

In an embodiment, the vent hole has a slit-shaped cross section, or anelliptical cross section. Such vent hole has a first dimension in thelongitudinal direction of the slit or ellipse, i.e., along the majoraxis, and a second dimension substantially perpendicular to thelongitudinal direction, i.e., along the minor axis. The first dimensionis then larger than a dimension across the charged particle beamtransmitting apertures, i.e., the diameter of the charged particletransmitting aperture. In some embodiments, also the second dimension islarger than the diameter of the charged particle transmitting aperture.

In an embodiment, the vent hole is larger than the charged particletransmitting aperture. For example, in an embodiment with a circularvent hole, the diameter of the vent hole may be a factor 5, or 10,larger than a diameter of the charged particle transmitting aperture. Inan example, the charged particle transmitting apertures have a diameterof 12 μm, at least on the upstream side of the substrate, and the ventholes have a diameter of 50 or 60 μm, or even up to 300 μm, or any valuethere between. The vent holes are separate holes, not intended totransmit charged particles.

In an embodiment, the charged particle transmitting apertures arearranged in one or more groups, and the vent holes are arrangedsubstantially along said one or more groups.

In an embodiment, said vent holes are arranged in one or more onedimensional arrays.

In an embodiment, said vent holes are arranged in one or moretwo-dimensional arrays. The vent holes may be arranged in a regularrectangular lattice. Alternatively, the vent holes may be arranged in apattern where the rows or columns of vent holes are shifted with respectto one another, for example forming a skewed array.

In an embodiment, said vent holes are arranged on either sides of saidplurality of charged particle transmitting apertures. In general, theplurality of charged particle transmitting apertures is arranged insubstantially one or more rectangular groups, or arrays, having a firstdimension which is larger than a second dimension. The vent holes arepreferably arranged along the long sides of the one or more groups ofcharged particle transmitting apertures.

In an embodiment, said vent holes are arranged immediately adjacent anarea comprising a plurality of said charged particle transmittingapertures.

In an embodiment, said vent holes are arranged with a pitch which isequal to or larger than a dimension of said vent holes, said pitch inparticular being in the range from 1 to 3 times the dimension of saidvent holes.

In an embodiment, the pitch is equal to or larger than a dimension ofthe vent holes along a direction of alignment of the vent holes. Asdescribed above, the vent holes may be arranged along one or more groupsof charged particle transmitting apertures. The vent holes may then bearranged in rows, arranged with a pitch which is equal to or larger thana dimension of the vent holes in the direction of the row.

In an embodiment, the system is arranged such that any charged particlespassing through a vent hole are prevented from reaching the target.Although the one or more vent holes are preferably arranged outside theintended charged particle beam trajectory, one or more elements orcomponents may be arranged downstream of the charged particle opticalelement to block a further path of any charged particles transmittedthrough a vent hole. Alternatively, an element or component might beprovided upstream the vent hole, for preventing charged particles fromreaching the vent hole.

In an embodiment, the charged particle optical element comprises a beamstop element, the beam stop element comprising:

-   -   a plurality of charged particle transmitting apertures for        passage of charged particle beams, and a non-aperture area for        blocking passage of charged particles, and    -   a plurality of vent holes for providing a flow path through the        beam stop element, the system further comprising    -   a projection lens comprising a plurality of projection lens        apertures for focusing the charged particle beams, wherein the        projection lens is arranged downstream the beam stop element,        and wherein the projection lens and the beam stop element are        arranged such that any charged particles passing through one or        more of the vent holes are blocked by a non-aperture area of the        projection lens.

Providing the beam stop element with vent holes has been seen to reduceaccumulation of contamination at the beam stop apertures. The vent holesenable a flow path from the target, through the projection lens andthrough the beam stop element and further towards a vacuum pump. Theprojection lens apertures are generally arranged in groups or arrayscorresponding to the charged particle transmitting apertures of the beamstop element. A scanning deflector is typically arranged between thebeam stop element and the projection lens, for scanning the chargedparticle beams over a portion of the target surface.

In an embodiment, the projection lens further comprises a plurality ofdummy apertures arranged around a group of the projection lensapertures, wherein the vent holes are arranged such that any chargedparticle passing through the vent holes are blocked by an area locatedlaterally outside the dummy apertures. By this arrangement, chargedparticles are efficiently prevented from reaching the target via a venthole. The dummy apertures are generally included to provide a similarelectrostatic field for all charged particle beams passing through theprojection lens. The dummy holes themselves do not provide a passage forcharged particle beams.

In an embodiment, the system further comprises:

-   -   a second aperture element, comprising a plurality of apertures        for forming a plurality of charged particle beams from the beam        of charged particles, the second aperture element arranged        between the charged particle beam generator and the charged        particle optical element, and    -   a restriction element provided between the charged particle beam        generator and the second aperture element, the restriction        element arranged for preventing or at least reducing a flow of        the cleaning agent or products thereof to the charged particle        beam generator.

The restriction element enables introduction of the cleaning agent whilethe beam generator is active. This allows introducing the cleaning agentinto the system even during target exposure. The beam generator maycomprise a charged particle source which requires a high vacuum duringoperation and which is sensitive to the presence of the speciescomprised in and/or formed from the cleaning agent. For example, athermionic cathode, which is often used as electron sources, is damagedif operated at too high partial oxygen pressure and/or in the presenceof oxygen radicals or ozone. Therefore, in order to be able to introducecleaning agent while the beam generator is active, it is important to atleast limit the flow of cleaning agent, or products or componentsthereof, to the beam generator. As mentioned, the cleaning agentpreferably comprises molecular oxygen and oxygen atomic radicals, and/orozone. Both oxygen atomic radicals and ozone molecules generallyrecombine to molecular oxygen along their flow path within the chargedparticle optical column. Therefore, at the beam generator, gasoriginating from the cleaning agent will comprise mainly molecularoxygen. For some systems, at least to some extent, the pressure at thecharged particle source can be sufficiently limited by differentiallypumping a space comprising the source. However, additional sealing orrestriction of a flow path to the beam generator, as described above,may be advantageous. The restriction element is not necessarily asealing element which substantially completely blocks a flow of gaseousspecies. What is important is that the charged particle source ismaintained in a vacuum which is within the operable range of thespecific source. A reason not to use a sealing element which completelyblocks the flow is the force which must be asserted on the sealingelement in order to achieve efficient sealing. Such forces might requiremodifications to an existing system if adding such sealing element.

In an embodiment, the charged particle beam system further comprises:

-   -   a beam generator module, the charged particle beam generator        being arranged in the beam generator module;    -   a modulation module, the second aperture element being arranged        in the modulation module;

wherein the restriction element is movably connected to the beamgenerator module and arranged abutting the modulation module by means ofgravity and/or a spring force. Flow paths of cleaning agent, or othergaseous species, into the beam generator module, is thereby limited totaking place either through the apertures of the second aperture elementor via the outside of the charged particle optical column, through therestricted flow path between the restriction element and the surface ofthe modulation module onto which the restriction element rests. Themodules may be provided as removable modules, arranged in a frame of thesystem. Arranging the restriction element to abut or rest on themodulation module by means of gravity limits the force exerted on themodulation module by the restriction element, while restricting the flowpath into the beam generator.

In an embodiment, the restriction element is connected to a first wallof the beam generator module, the restriction element at least partlysurrounding a perimeter of an opening in the first wall for passage ofthe beam of charged particles, wherein the restriction element comprisesan at least partially ring-shaped element, in particular a ceramic ring,the at least partially ring-shaped element being movably arranged withrespect to the first wall in a direction toward or away from themodulation module.

In an embodiment, the system further comprises a confining element forconfining a movement of the ring-shaped element with respect to thefirst wall.

In an embodiment, the confining element is made of a material comprisingaluminum or titanium.

In an embodiment, the ring-shaped element is loosely arranged at leastpartly within a groove or recess within the first wall, and is preventedfrom falling out by the confining element.

In an embodiment, the restriction element is provided with one or moreprotrusions and the confining element is arranged to cooperate with theprotrusions to confine movement of the restriction element. Such flowrestricting arrangement enables easy removal and/or replacement of thebeam generator module, while maintaining the specified flow restriction.In an embodiment, the confining element has an at least partial ringshape.

The flow restricting arrangement, comprising the restriction element andthe confining element, is designed such that it does not influence theelectromagnetic field within the system, and thus, does not influencethe charged particle beam paths through the column.

In an embodiment, the system further comprises:

-   -   a modulation element arranged downstream the second aperture        element, the modulation element comprising a second plurality of        apertures for passage of the charged particle beams and a second        plurality of deflectors associated with the second plurality of        apertures, the deflectors arranged to selectively deflect or not        deflect the charged particle beams, and    -   a beam stop element comprising a third plurality of apertures        for passage of charged particle beams and a blocking area for        blocking charged particle beams, the beam stop element arranged        downstream the modulation element,        the modulation element and the beam stop element arranged to        function together to let pass or to block the selectively        deflected charged particle beams,        wherein the conduit is arranged to direct the cleaning agent        toward the beam stop element and, preferably, also toward the        modulation element. Contamination of the beam stop element can        thereby be prevented or at least removed. The beam stop element        represents a charged particle optical element as described        above. By blocking charged particle beams, these are prevented        from continuing along the trajectory toward the target. This        beam stop element may be the beam stop element described above.        Each aperture of the modulation element may be provided with a        deflector. The modulation element, also referred to as blanker,        can thereby deflect one or more individual charged particle        beams, while not deflecting other individual beams, in        accordance with pattern data.

In an embodiment, electrical connections within the charged particleoptical system are provided with a protective coating, such as epoxyand/or a metal layer. Such protective coating prevents electricalconnections, such as conducting wires, electrical contacts, contactpads, etc., from being damaged by the cleaning agent or species thereof.

In an embodiment, a second charged particle beam generator is provided,arranged such that a beam of charged particles emitted therefrom isdirected toward, along or over the charged particle optical element or asurface thereof, but does not reach the target. The provision of theadditional charged particle beam generator, generating electrons orions, may enhance prevention or removal of contamination. It alsoenables cleaning facilitated by charged particles, also when the chargedparticle beam generator provided for target exposure is not active.

One or more of the various features of the above described embodimentsmay be combined.

According to a second aspect, the invention provides a method forpreventing or removing contamination of a charged particle transmittingaperture in the charged particle beam system according to any one of theembodiments of the first aspect, the method comprising the steps of:

-   -   introducing a cleaning agent towards the charged particle        optical element while the beam generator is generating a beam of        charged particles and/or while a second charged particle beam        source is generating a beam of charged particles which is        directed toward the charged particle optical element; and    -   maintaining a vacuum in the vacuum chamber while introducing the        cleaning agent, wherein the step of maintaining a vacuum        comprises providing a flow at least through the charged particle        optical element via the vent hole to a vacuum pump connected to        the vacuum chamber.

The method enables preventing or at least limiting deposition and growthof contamination in or near charge particle transmitting apertures, aswell as removal of contamination formed on surfaces. The aperture can bemaintained in an open state, i.e. its size maintained, and the shape ofthe aperture can be maintained. Thereby, charged particle beamproperties such as current density, shape, and position are maintained.As discussed above with respect to the first aspect, the vent holeprovides a reduction in pressure at the charged particle opticalelement. Introducing cleaning agent while the beam generator is activemight seem contra intuitive, since, as discussed above, charged particlebeams may interact with species to form deposits onto surfaces. However,the inventors have observed that introducing the cleaning agent in thepresence of charged particle beams leads to more efficient cleaning ofcharged particle optical elements. The inventors have seen thatintroducing the cleaning agent while the charged particle beam generatoris active, i.e., switched on, improves the cleaning rate compared to themethod disclosed in US 2015/028223 A1, which was applied when the beamgenerator was not active. In particular, efficient removal or preventionof contamination has been observed in charged particle optical elementscomprising apertures through which the charged particle beams aretransmitted at a very close distance to the perimeter of the apertureand/or where charged particle beams are, at least partly, blocked by thearea surrounding the aperture. Such current limiting apertures aretypically provided in aperture elements forming a plurality of beamsfrom an incoming beam, in beam forming or beam shaping elements, incharged particle beam modulation elements (blankers), or chargedparticle beam blocking elements (beam stops). The charged particle beamstravelling through at least a portion of the charged particle opticalcolumn enable cleaning at specific locations. It has been seen that,using the method of the second aspect, cleaning can be performed at arate higher than the rate at which contamination accumulates on thesurface. Thereby, a steady state is achieved in which the level ofcontamination is at least substantially constant in time. An increasedstability of the charged particle optical column has been observed whenapplying the method substantially continuously. This is considered to berelated to, e.g., the absence of transitions between clean andcontaminated states of charged particle optical surfaces.

The method has been seen not to interfere with the normal operation ofthe system. The method may be performed, e.g., during preparation orexchange of targets, and/or during exposure of targets to the chargedparticle beams, e.g. during lithographic patterning of wafers.

In an embodiment, the cleaning agent is introduced substantiallycontinuously, during operation of the beam generator. This facilitatessubstantially continuous removal of contamination, or prevention ofgrowth of contamination on charged particle optical elements.

In an embodiment, maintaining vacuum comprises actively operating one ormore vacuum pumps connected to the vacuum chamber.

In an embodiment, the cleaning agent is directed to charged particleoptical elements comprising one or more current limiting apertures,and/or to elements where contamination may influence or limit the properfunctioning of the elements, e.g. where a lifetime of the component mayotherwise be limited by contamination.

In an embodiment, the method comprises the step of preventing anycharged particles passing through the at least one vent hole fromreaching the target.

In an embodiment, the charged particles passing through a vent hole areprevented from reaching the target by blocking these charged particlesby non-aperture areas comprised in a further aperture element arrangeddownstream the charged particle optical element, the further apertureelement comprising one or more apertures for passage of charged particlebeams having passed through the charged particle transmitting apertures.Such aperture element may be comprised in the projection lens describedabove. Alternatively or additionally, the charged particles may beblocked upstream the vent holes, preventing charged particle beams fromreaching the vent holes.

In an embodiment, the method further comprises the step of:

-   -   arranging the charged particle beam system such that a flow of        the cleaning agent or products thereof to the charged particle        beam generator is prevented or at least reduced.

In an embodiment, the method further comprises the following steps:

-   -   arranging the charged particle beam generator in a beam        generator module and the charged particle optical element in a        modulation module,    -   providing a restriction element, movably connected to the beam        generator module, and abutting the modulation module by means of        gravity and/or spring force. The restriction element may be a        restriction element as described above.

In an embodiment, the method comprises introducing the cleaning agent ina region of the charged particle optical column where the chargedparticles have energy in the range of 1-10 kEV, in particular around orlower than 5 keV. The cleaning agent can hence be introduced duringnormal exposure. The energy of the charged particles is determined byelectrical potentials applied to, e.g., the charged particle beamsource, the target, and the charged particle optical elements within thesystem. In a multi-beam system as described herein, to which the methodis especially suited, the energy of the charged particles is typicallyaround 5 keV during target exposure. If the method is applied while notexposing a target, the energy of the charged particle beam can beadjusted to improve cleaning.

In an embodiment, one or more charged particle beams is present at ornear the charged particle optical element while directing the cleaningagent toward the charged particle optical element. The term “at or near”comprises transmitting charged particle beams towards the chargedparticle optical element and/or through the charged particletransmitting aperture, i.e., the charged particle beams being at leastpartly transmitted through at least a portion of the charged particleoptical column.

During introduction of the cleaning agent the pressure is maintained ata level where the mean free path of the species of the cleaning agent issuch that the species most likely collide with a surface of a chargedparticle optical element, or be pumped away from the system, without anyother collisions, in particular with a charged particle of the chargedparticle beams. The cleaning agent is therefore virtually invisible tothe charged particle beams, which are therefore not influenced by thepresence of the cleaning agent. Furthermore, the pressure in the systemis maintained below a pressure at which there might be a risk ofelectrical breakdown, or flashover, between charged particle opticalelements.

One or more of the various features of the above described embodimentsmay be combined. The method according to the second aspect may beperformed or applied in a charged particle beam system according to anyone or more of the embodiments or alternatives of the first aspect. Thevarious embodiments of the method, in particular the different steps,may be realized by one or more of the features of the charged particlebeam system of the first aspect.

According to a third aspect, the present invention provides a method forpreventing or removing contamination of a charged particle transmittingaperture in a charged particle beam system arranged in a vacuum chamber,the charged particle beam system comprising a charged particle opticalcolumn for projecting a beam of charged particles onto a target, thecharged particle optical column comprising a charged particle opticalelement for influencing the beam of charged particles,

the charged particle optical element comprises the charged particletransmitting aperture for transmitting and/or influencing the beam ofcharged particles, and at least one vent hole for providing a flow pathfrom a first side to a second side of the charged particle opticalelement;the method comprising the following steps:

-   -   introducing a cleaning agent towards the charged particle        optical element while a beam of charged particles is present at        or near the charged particle optical element; and    -   maintaining a vacuum in the vacuum chamber,        wherein the step of maintaining a vacuum comprises reducing a        pressure on the first side of said charged particle optical        element by providing a flow through the vent hole, from the        first side to a to a second side of the charged particle optical        element and further to a vacuum pump connected to the vacuum        chamber.

The method according to the third aspect provides the same orcorresponding advantages as described above for the second aspect. Themethod of the third aspect may comprise any one or more of the features,alternatives, or method steps of the embodiments described above withrespect to the method of the second aspect.

According to a fourth aspect, the invention provides a charged particlebeam system, comprising:

-   -   a charged particle beam generator for generating a beam of        charged particles;    -   a charged particle optical column arranged in a vacuum chamber,        wherein the charged particle optical column is arranged for        projecting the beam of charged particles onto a target, and        wherein the charged particle optical column comprises a charged        particle optical element for influencing the beam of charged        particles;    -   a source for providing a cleaning agent;    -   a conduit connected to the source and arranged for introducing        the cleaning agent towards the charged particle optical element;        wherein the charged particle optical element comprises a charged        particle transmitting aperture for transmitting and/or        influencing the beam of charged particles,    -   a second aperture element, comprising a plurality of apertures        for forming a plurality of charged particle beams from the beam        of charged particles, the second aperture element arranged        between the charged particle beam generator and the charged        particle optical element, and    -   a restriction element provided between the charged particle beam        generator and the second aperture element, the restriction        element preventing or at least minimizing a flow of the cleaning        agent and/or products thereof to the charged particle beam        generator.

The restriction element prevents or at least restricts a flow ofcleaning agent to a charged particle source arranged in the beamgenerator. Thereby, cleaning agent can be introduced during normaloperation of the system, providing efficient cleaning and/or reducingdown time of the system.

The system of the fourth aspect may comprise or be combined with any oneor more of the features or alternatives of the embodiments describedabove with respect to the system of the first aspect.

In a fifth aspect, the invention provides a method for preventing orremoving contamination of a charged particle transmitting aperture in acharged particle optical element in a charged particle beam systemaccording to the fourth aspect,

the method comprising the steps of:

-   -   introducing the cleaning agent towards the charged particle        optical element while the beam generator is generating the beam        of charged particles and/or while a second charged particle beam        source is generating a beam of charged particles which is        directed toward the charged particle optical element; and    -   maintaining a vacuum in the vacuum chamber while introducing the        cleaning agent, wherein the charged particle beam system is        arranged such that a flow of the cleaning agent or products        thereof to the charged particle beam generator is prevented or        at least minimized.

The method according to the fifth aspect provides the same orcorresponding advantages as described with respect to the fourth aspect.The method of the fifth aspect may comprise or be combined with any oneor more of the features or alternatives of the embodiments describedabove with respect to the method of the second and third aspect. Themethod steps may be realized by any one or more of the features of theembodiments described with respect to the first and/or fourth aspect.

According to a sixth aspect, the present invention provides a chargedparticle beam system, comprising:

-   -   a charged particle beam generator for generating a beam of        charged particles;    -   a charged particle optical column arranged in a vacuum chamber,        wherein the charged particle optical column is arranged for        projecting the beam of charged particles onto a target, and        wherein the charged particle optical column comprises a charged        particle optical element for influencing the beam of charged        particles;    -   a source for providing a cleaning agent;    -   a conduit connected to the source and arranged for introducing        the cleaning agent towards the charged particle optical element;        wherein the charged particle optical element comprises a charged        particle transmitting aperture for transmitting and/or        influencing the beam of charged particles, and at least one vent        hole for providing a flow path between a first side and a second        side of the charged particle optical element,        wherein the vent hole are arranged outside an intended        trajectory for the beam of charged particles.

The system of the sixth aspect provides analogous advantages and effectsas the system of the first aspect. The system of the sixth aspect maycomprise any one or more features of the embodiments described abovewith respect to the systems of the first and fourth aspects. A method,as defined by the method steps of the second or third aspect, may beapplied to the system of the sixth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the systems and methods will be further explainedwith reference to embodiments shown in the drawings.

FIG. 1 schematically shows a multiple beam lithography system;

FIGS. 2A and 2B schematically illustrate charged particle beam induceddeposition and contamination growth at a charged particle transmittingaperture;

FIG. 3A schematically illustrates a charged particle beam systemaccording to embodiments of the present invention;

FIG. 3B schematically illustrates a detail of FIG. 3A;

FIG. 4A-4D schematically show details of charged particle opticalelements comprising charged particle transmitting apertures and ventholes;

FIG. 5 schematically shows a detail of an element arranged downstreamthe charged particle optical element illustrated in FIG. 4A, in acharged particle beam system;

FIG. 6 schematically illustrates an arrangement for restricting a flowpath into a beam generator module of a charged particle beam system;

FIG. 7 schematically shows an arrangement for introduction of a cleaningagent into a charged particle beam system;

FIG. 8 schematically illustrates a cleaning agent source;

FIGS. 9A and 9B schematically illustrate a method for preventing orremoving contamination in a charged particle beam system;

FIG. 10 schematically illustrates a charged particle beam systemaccording to embodiments of the invention.

DESCRIPTION

Various embodiments of charged particle beam systems and methods forpreventing or removing contamination of charged particle transmittingapertures such systems are described below, given by way of example onlyand with reference to the figures.

FIG. 1 shows a simplified schematic drawing of an embodiment of acharged particle multi-beam lithography system. Such lithography systemis described in U.S. Pat. Nos. 6,897,458; 6,958,804; 7,019,908;7,084,414; 7,129,502; 7,709,815; 7,842,936; 8,089,056 and 8,254,484; andin U.S. patent application publication nos. 2007/0064213; 2009/0261267;US 2011/0073782 and US 2012/0091358, assigned to the applicant of thepresent application and hereby incorporated by reference in theirentirety. The same holds true for the embodiment provided in US2014/0197330, in which the embodiment illustrated in FIG. 1 provides afunctionally equivalent system. Advantageous mounting arrangements,suspension mechanisms and vibration isolation arrangements are alsodescribed in US 2014/0197330, which may also be combined or used in thesystem described in systems of the above listed publications. Althoughthe lithography system is described with reference to electron beams,the teaching applies to other types of charged particle beams as well,such as ion beams. The term “electron” is in that case replaced by“charged particle” or “ion”, as understood by the skilled person. Themulti-beam lithography system 1 illustrated in FIG. 1 comprises a vacuumchamber 2, comprising an electron source 4 and an electron opticalsystem 6 for forming and controlling electron beams 8 for patterning asurface 10 of a target 12. The target 12 typically comprises a siliconwafer coated with an electron sensitive resist layer. The electronsource 4 and the components of the electron optical system 6 are alignedalong an optical axis 14. The electron optical system is also referredto as a charged particle optical column. The components of the electronoptics, which will be described in more detail below, are advantageouslyarranged in one or more replaceable modules, supported by a frame 7. Theframe and/or the modules may be configured for providing alignment ofthe modules along the optical axis 14. Although a specific division orarrangement into different modules is described herein, this should notbe construed as limiting, since other arrangements are also possible.

A beam generator module 16 comprising the electron source 4 and a beamcollimating system 18 generates a collimated electron beam 20. Thecollimated electron beam 20 is divided into a plurality of individualbeams 8 in an aperture array and condenser lens module 22. Thecollimated beam 20 is divided into a plurality of beams by an aperturearray element (second aperture element 23), comprising one or moregroups or arrays of apertures. The beams 8 are further directed to abeam blanker 24, also referred to as modulation element, configured toselectively blank, i.e., deflect or not deflect, individual beams 8, inaccordance with pattern data. In some embodiments, a multi-aperturearray (not shown) is provided between the aperture array element and thebeam blanker array, or is arranged integral with the beam blanker. Suchmulti-aperture array is arranged for further splitting each of the beams8 into smaller beams, which are directed to the beam blanker in groups.Patterned beams can be formed by individually modulating the beamswithin a group of beams 8. The beam blanker 24 may be arranged in amodulation module 25, also referred to as beam switching module.Alternatively, the aperture array element, the multi-aperture array (ifprovided), and the blanker array may be arranged in the same module.

A beam stop element 26 is arranged to stop beams 8 which are deflectedby the beam blanker 24. Electron beams 8 which are not deflected by theblanker array 24 are transmitted through the beam stop element 26. Thebeam blanker 24 and the beam stop element 26 thus function together tomodulate the beams, by stopping or allowing individual electron beams 8to pass. In some embodiments the beam stop element is arranged in aprojection optics module 28. This module also comprises a deflectorarray (scanning deflector) and a projection lens (not illustrated inFIG. 1). The scanning deflector deflects beams 8 in order to scan themover respective writing areas, stripes, on the surface 10. Theprojection lens focuses the beams 8 onto the target surface 10. A detailof the projection optics 28 is shown in FIG. 3B, schematically showingan arrangement of beam stop element 26, scanning deflector 27, andprojection lens 29. The beam blanker 24, the beam stop element 26, andthe projection lens 29 are formed as aperture elements, comprising aplurality of charged particle beam transmitting apertures, preferablyarranged in one or more arrays.

The target 12 is supported by a target support 30, here a wafer table 32mounted on a chuck 34. A target support actuator 36 is provided formoving the target support 30 with respect to the electron optical column6, in particular with respect to the electron optical axis 14. Theactuator 36 may comprise a short stroke actuator 38 and a long strokeactuator 40, enabling two-dimensional movement of the target in a planeperpendicular to the electron optical axis, with high accuracy.

A lithography control unit 42 is configured to control the operation ofthe lithography system. Pattern data is transmitted from the controlunit 42 to the modulation element 24. In an embodiment, a part of thedata transmission is realized by modulated light beams directed ontoreceivers on the modulation element. The control unit 42 may alsocontrol and/or use various measurements, such as measurements ofelectron beam properties and measurements of the position of the targetwith respect to the projection optics, for controlling operation of thesystem.

At least one vacuum pump 44 is connected to the vacuum chamber 2, inorder to maintain the desired vacuum therein. Typically one or moreturbo pumps are used. Further, one or more pumps, typically a (ion)getter pump is connected to the beam generator, in order to maintain aspecified vacuum herein. The pressure within the beam generator moduleis generally lower than the pressure in the main vacuum chamber.Alternatively, the beam generator module is arranged in a separatevacuum chamber connected to the main vacuum chamber.

The teachings of the present invention, embodiments of which aredescribed below, can be applied as modifications to the system ofFIG. 1. As illustrated in FIG. 7 and FIG. 10, a source of cleaning agentcan be connected to the frame 7, and provided with one or more conduitsfor directing the cleaning agent toward one or more of the chargedparticle optical elements, for example the modulation element 24 and/orthe beam stop element 26, 226. One or more of the elements comprisingarrays of charged particle transmitting apertures, in particular thebeam stop 226, is provided with vent holes in addition to the chargedparticle transmitting apertures, as illustrated in FIG. 3A and FIG. 9B.In some embodiments a restriction element is provided between the beamgenerator module 16 and the aperture array module 22 or the combinedaperture array and blanker module 225, as described with reference toFIG. 6. However, the teachings of the present invention are not limitedto the type of system illustrated in FIG. 1, but can be applied to othertypes of charged particle beam systems as well.

FIGS. 2A and 2B schematically illustrate formation of contaminationcaused by charged particle beam induced deposition (EBID or IBID) at acharged particle beam transmitting aperture 46. Such apertures arepresent in various components in a charged particle beam system. In themulti electron beam lithography system illustrated in FIG. 1, currentlimiting charged particle transmitting apertures are present in theaperture array 22, the multi-aperture array (if provided), the blanker24, and in the beam stop 26.

FIG. 2A shows a detail of a charged particle optical element 48 forinfluencing one or more charged particle beams 8, at least some of whichare directed toward a substrate 12. The charged particle optical element48 is provided with at least one charged particle transmitting aperture46, as illustrated in FIG. 2A. Residual gases, or contaminants 50, are,at least to some degree, present in the vacuum system. Such contaminants50 can originate from resist outgassing, illustrated by reference number52, typically providing hydro-carbon compounds or molecules(C_(x)H_(y)). Other sources of contaminants 50 are outgassing fromsurfaces within the charged particle column itself. Hydrocarbons orother molecules 50 may adsorb onto a surface of the charged particleoptical element 48, illustrated by reference number 54. Chargedparticles in the charged particle beams 8, which pass by at very closedistance of or even, at least partially, hit, the border of the aperture46, may interact with residual gases present in the vicinity of oradsorbed on the surface, thereby causing charged particle beam induceddeposition (EBID, IBID). Upon this interaction bonds within the moleculemay break, whereby volatile parts of the molecule 50 are pumped away bythe vacuum pumps. The remaining part of the molecule, comprising inparticular carbon, remain on or near the surface, where they may form alayer 56. The layer 56 of carbon containing material influences beamstability, for example due to charging of components, and may causeintensity loss of charged particle beams projected onto the targetsurface, and/or distortions such as aberrations, etc. As illustrated inFIG. 2B, build-up of such contamination layer 56 in and around apertures46 reduces the size of the apertures 46. As the contamination layer 56grows, for example due to continued electron or ion beam induceddeposition during operation of the charged particle beam system, theeffective aperture becomes smaller and smaller, and eventuallyeffectively totally clogged.

FIGS. 3A and 3B illustrate a charged particle optical system, or atleast components thereof, according to an aspect of the invention. Thevarious features illustrated in FIGS. 3A and 3B may substitutecorresponding features of the prior art system of FIG. 1, and/or may beadded to the system of FIG. 1. The teaching can also be applied to othertypes of systems. In the figures, elements denoted with the samereference number are similar as described above,

FIG. 3A illustrates a charged particle beam system 201, comprising acharged particle beam generator 16, for generating a beam 20 of chargedparticles, and a charged particle optical column 206 arranged in avacuum chamber 2. The charged particle optical column 206 is arrangedfor projecting one or more charged particle beams 8, formed from thebeam 20 of charged particles, onto a target 12. The charged particleoptical column 206 comprises charged particle optical elements forinfluencing the charged particle beams. In the embodiment illustrated inFIG. 3A, the system comprises charged particle optical elements in theform of a modulation element 24, a beam stop element 226, and aprojection lens 29, which can be analogous in function to thecorresponding elements described with reference to FIG. 1. In order tomaintain the vacuum in the vacuum chamber 2, one or more vacuum pumps 44are provided.

The modulation element 24 comprises a plurality of apertures 46 forpassage of said charged particle beams and a corresponding plurality ofdeflectors, or electrodes, each associated with an aperture. Thedeflectors are arranged to selectively deflect or not deflect one ormore charged particle beams. The beam stop element 226, arrangeddownstream the modulation element, comprises a plurality of apertures 46for passage of charged particle beams, and a blocking area, typicallythe surface adjacent the apertures, for blocking charged particle beams.The modulation element and the beam stop element are configured tocooperate to let pass or not let pass, i.e. block, the selectivelydeflected charged particle beams. In both the modulation element 24 andthe beam stop element 226 the apertures 46 may act as current limitingapertures.

As described above, species, such as hydrocarbons, contained in theresist layer 10 may be released therefrom. These species may travelfurther within the system, as determined by conductance values alongdifferent flow paths and pumping speeds within the system, ultimatelytoward the vacuum pump 44. As illustrated in FIG. 3A, a molecule orcluster 50 present in the space between the target surface 10 and thecharged particle optical column 206 can flow either along a pathindicated by arrow F1, leading substantially radially, to the exteriorof the charged particle optical column, or, as indicated by arrow F2,into the charged particle optical column 206 via projection lensapertures 58. In some systems the distance d between the target surface10 and the part of the charged particle optical column closest to thetarget surface is very small. For example, in the systems illustrated inFIG. 1, this distance is around 50 μm (micrometers), whereas theapertures of the projection lens array typically have a diameter of 100μm, that is, a diameter of comparable or even larger dimension.Contaminants 50 may thereby experience a comparable or even lower flowresistance along path F2 than along path F1. This may lead to arelatively high partial hydrocarbon pressure in at least parts of thecharged particle optical column. In a system as illustrated in FIG. 1,the charged particle beams 8 travels through the projection lensapertures 58 at a distance to their perimeter. Therefore, in principle,the projection lens apertures 58 are relatively insensitive to chargedparticle beam induced deposition. The beam stop element 226, however,forms a current limiting element and is therefore sensitive to chargedparticle beam induced deposition if being subjected to presence ofcontamination species. Therefore, it is desired to avoid accumulation ofcontaminant species in the area around the beam stop element.

In order to address the problems associated with contamination of theapertures 46 of the beam stop element 26, according to the invention thebeam stop element 226 is provided with a plurality of vent holes 60.These vent holes enable contaminant species to flow from a downstreamside of the beam stop 226, facing the projection lens 29, through thebeam stop to an upstream side of the beam stop, and subsequently exitthe charged particle optical column, as indicated by arrow F3. Thereby,the vent holes provide a manner of reducing the pressure ofcontamination species at the beam stop apertures 46, hence reducing theamount of material which may cause contamination of the charged particleoptical element. Buildup of pressure, in particular of contaminantspecies, at the charged particle optical element is hence prevented orat least minimized. This in turn reduces contamination in or at thecharged particle transmitting apertures. As illustrated in FIG. 3A, thecross section of each vent hole is larger than the cross section of anindividual charge particle transmitting aperture.

The system illustrated in FIG. 3A further comprises a source 62providing cleaning agent and conduits 64 directing the cleaning agenttowards the modulation element 24 and the beam stop element 226.Directing the cleaning agent toward these elements 24, 226 enablescleaning, which, as observed by the inventors, is enhanced by thepresence of charged particle beams. In this way, cleaning can bedirected to the locations most prone to contamination. Contaminantspresent as gases, species of the cleaning agent, and products formed byreactions between the cleaning agent and contamination layers 56 mayexit the charged particle optical column 206 and be pumped away by thevacuum pump 44.

FIG. 3B illustrates a detail of the projection lens module 28, 228 usedin the systems of FIG. 1 and FIG. 3A, respectively. The detail of FIG.3B shows one beam stop aperture 46 and one projection lens aperture 58,out of the plurality of apertures in the arrays. As schematicallyindicated, the charged particle transmitting aperture 46 of the beamstop element 26, 226 is typically smaller than the projection lensaperture 58. While the beam stop aperture 46 blocks part of the chargedparticle beam 8, the projection lens is configured such that the chargedparticle beam 8 not intentionally contacts the projection lens 29. Theprojection lens typically comprises three lens elements 29 a, 29 b, 29 cfocusing the charged particle beam transmitted through the lens,although other configurations might also be possible. Between the beamstop element 26, 226 and the projection lens 29, a scanning deflector 27is provided, for scanning the charged particle beam over an area of thetarget surface 10. The projection lens apertures 58 are arranged incorrespondence with the charged particle transmitting apertures 46 ofthe beam stop element 26, 226.

FIG. 4A-4D each shows a detail of a charged particle optical elementprovided with a plurality of charged particle transmitting apertures 46,intended for allowing passage of charged particle beams, and a pluralityof vent holes 60, for enabling passage of gaseous species. As indicatedin FIG. 3A, and as can also be seen in FIG. 9B, the charged particleoptical element preferably comprises a substantially flat substrate, inwhich the vent holes 60 are provided by a plurality through-holesextending through the substrate. In the embodiments illustrated in FIG.4A-4D, the charged particle transmitting apertures 46 are arranged inone or more arrays 68 (of which one is shown in FIG. 4A-4D) and the ventholes 60 are arranged adjacent such array 68 of charged particletransmitting apertures. In the illustrated embodiment, the vent holesare arranged directly adjacent the array 68. The aperture array 68extends in a two-dimensional array, substantially along the width of thecharged particle optical element, in one or more beam areas. The ventholes may be provided on either or both sides of the group or array 68of charged particle transmitting apertures, in particular in one or moreof the non-beam areas. As can be seen in FIG. 4A-4D, the vent holes aregenerally located separate from the charged particle transmittingapertures. The locations of the vent holes are chosen such that to it isunlikely that charged particle will pass through the vent holes duringnormal operation of the system. Furthermore, one or more blockingelements can be provided, to either prevent charged particles fromentering the vent hole, or for blocking a further passage of a chargedparticle which has passed through a vent hole.

As also illustrated in FIG. 4A-4D, the vent holes are generally largerthan the charged particle transmitting apertures. For example, inembodiments with circular vent holes, the diameter of the vent holes maybe a factor 5, or 10, larger than the diameter of the charged particletransmitting apertures. In some embodiments, the charged particletransmitting apertures have a diameter of 12 μm, at least on theupstream side of the element, and the vent holes have a diameter of 50or 60 μm, or even up to 300 μm, or any value there between. The numberof vent holes provided can be related to their size.

The larger their diameter the less number of vent holes are required forachieving the flow path through the vent holes.

In the embodiment illustrated in FIG. 4A, one row of vent hole isprovided on either side of the charged particle transmitting apertures.The vent holes are arranged at regular distances to one another alongthe row. In the illustrated example, the pitch p is about twice thediameter of the vent holes.

Alternatively, a plurality of vent holes can be provided. The vent holescan hence be arranged in two-dimensional arrays. In FIG. 4B, the ventholes 60 are arranged in two rows on both sides of the array 68 ofcharged particle transmitting apertures 46. In the embodimentillustrated in FIG. 4B, the rows are arranged shifted a half pitch withrespect to one another.

In alternative embodiments, as illustrated in FIGS. 4C and 4D, the ventholes 60 a, 60 b have elongated shape, e.g. slit shape or ellipticalshape. In FIG. 4C, the vent holes 60 a are of slit shape, a plurality ofsuch vent holes being arranged along one row on either side of the array68. Alternatively, two or more such rows may be provided. In FIG. 4D, anembodiment is shown having thin slit shaped vent holes 60 b. These ventholes 60 b are thinner and longer than the vent holes 60 a of FIG. 4C.In FIG. 4D, one or more vent holes 60 b may be provided on either sideof the array 68 of charged particle transmitting apertures 48.

The number of vent holes 60, 60 a, 60 b, the cross section of the venthole 60, 60 a, 60 b, the pitch p between adjacent vent holes, and thearrangement of the vent holes, i.e., in one or two dimensional groups orarrays, as well as their distance to the charged particle transmittingapertures are chosen such that a flow path is created, and such that aspecified vacuum is obtained at the optical element.

FIG. 5 illustrates an arrangement for preventing passage of chargedparticles to the target surface via vent holes 60 provided in the beamstop element 226. FIG. 5 illustrates a portion of a surface area of aprojection lens 29, typically the upstream surface of the uppermostprojection lens electrode 29 a, facing the beam stop element 226. Aportion of an array of projection lens apertures 58 is shown, and anumber of dummy apertures 66 are located at borders of the array ofprojection lens apertures 58. The shaded areas 72 represent the positionof the vent holes 60 of the beam stop element 226 with respect to theapertures 58 of the projection lens. In other words, the areas 72illustrate a projection of the vent holes 60 on the projection lens 29.As can be seen, the vent holes are arranged such that any chargedparticle beam passing through the vent holes impinge on an area of theprojection lens not provided with apertures, in particular on an arealocated laterally outside the projection lens apertures, and, if dummyapertures are provided, laterally exterior of such dummy apertures. FIG.5 illustrates this for a vent hole arrangement as illustrated in FIG.4A. It should be clear, however, that in embodiments any of thearrangements shown in FIG. 4B, 4C or 4D are arranged such the vent holes60, 60 a, 60 b are positioned above areas laterally external to theprojection lens apertures, and if present laterally outside dummy holes70, hence giving rise to corresponding shadowed areas.

A further feature of the present invention is illustrated in FIG. 6. Inorder to prevent damage of the charged particle source, it is importantto maintain a specified degree of vacuum during its operation.Therefore, in order to be able to operate the beam generator whencleaning agent is present in the system it is important to prevent, orat least limit, a flow into the beam generator module. For a chargedparticle beam system as illustrated in FIG. 1, adding a restrictionelement restricting a flow path into the beam generator module has beenseen to be advantageous for maintaining proper functioning of thecharged particle source.

FIG. 6 illustrates an arrangement 74 which, at least to some extent,restricts or reduces a flow path from an exterior of the chargedparticle optical column 6, 206, herein represented by a module 22, intothe beam generator module 216. The charged particle beam systemcomprises a beam generator module 216 comprising a charged particlesource and possibly one or more charged particle optical elements, asdescribed above with reference to FIG. 1. In the embodiment of FIG. 6 anaperture array 23, also referred to herein as second aperture element isprovided, which comprises a plurality of apertures 66 for forming theplurality of charged particle beams 8 from the beam 20 of chargedparticles emitted by the beam generator. The restriction arrangement 74is provided between the charged particle beam generator module and thesecond aperture element 23 for preventing or at least minimizing a flowof cleaning agent or products thereof into the charged particle beamgenerator. The restriction element 76 is movably connected to the beamgenerator module 16 and arranged for abutting or resting on thedownstream module 22, or on a surface surrounding the aperture array 23.The applied force may result from only gravity, or may be provided by aspring blade, leaf spring, or similar. A flow into the beam generatormodule is thereby limited to taking place through the apertures 66 ofthe second aperture element 23 and/or via the outside of the chargedparticle optical column 206, through the restricted flow path betweenthe restriction element 76 and the surface of the element onto which therestriction element rests. The arrangement 74 can be applied to thesystem illustrated in FIG. 1 for providing a flow restriction betweenthe beam generator module 16 and the subsequent downstream module. Theflow restricting arrangement 74 can be incorporated in the system ofFIG. 1 substantially without any modifications, or only minormodifications, to the rest of the system.

In the embodiment illustrated in FIG. 6, the restriction element 76comprises a ring-shaped element surrounding an opening 80 provided in afirst wall 82, for passage of the beam of charged particles 20. Thering-shaped element is movably arranged partly within a recess in thefirst wall 82. A movement of the ring-shaped element 76 is confined by astop element, or confining element, 78. The ring-shaped element 76further comprises one or more protrusions 77, cooperating with theconfining element 78. Such flow restriction arrangement enables easyremoval and/or replacement of the beam generator module, whilemaintaining the specified flow restriction. Furthermore, the restrictionarrangement 74 does not influence the electric field within the system.

FIG. 7 schematically illustrates an embodiment of a cleaning agentsource 62 arranged in the frame 7 of the charged particle beam system,connected to conduits 64 for introducing the cleaning agent into thecharged particle optical column. Such arrangement can be used in thesystem illustrated in FIG. 3A, although other arrangement are alsopossible. Although in FIG. 7 the source is arranged within the vacuumchamber 2, alternatively the cleaning agent source 62 can be arrangedoutside the vacuum chamber, the conduit 64 extending into the vacuumchamber. The cleaning agent source and the one or more conduits may bean arrangement as illustrated in FIG. 8, which is known from US2015/028223 A1.

The arrangement 84 shown in FIG. 8 comprises a radio frequency (RF)plasma generator comprising a chamber 86 provided with an RF coil 88.The input gas, such as oxygen, which forms a precursor for the radicals,is introduced through the inlet 90. The plasma, gas molecules and/orradicals leave the chamber 86 via one or more outlets 92. In theembodiment illustrated in FIG. 8, the outlets 92 are provided by aplurality of apertures 92 are provided in an arrayed plate or similar.The provision of such aperture plate has however been observed not to beessential. Such plate may be disposed of, the outlet 92 provided by thefunnel 94. The arrangement 84 further comprise a pressure regulator,such as a funnel 94, and the conduit 64, for focusing and guiding thecleaning agent created in the source toward the charged particle opticalelement. A valve or a pump 96, controlled by a control unit 100, can beprovided for introducing the input gas with a controlled flow into thechamber 86.

FIGS. 9A and 9B schematically illustrate prevention and/or removal ofcontamination of charged particle transmitting apertures 46 provided ina charged particle optical element 48 in a charged particle beam system.This method can be applied to charged particle beam systems describedabove with reference to FIG. 1, 3-6. FIG. 9A shows a charged particleoptical element 48 provided with a charged particle transmittingaperture 46, where a charged particle beam 8 impinge at the borders ofthe aperture 46. A contamination layer 56 has formed at the aperture 46,e.g. by interaction of the charged particle beam with contaminants.According to the present method, these contaminants are removed,indicated by reference numeral 57, by introducing the cleaning agent 100in the presence of the charged particle beam 8.

FIG. 9B schematically illustrates the method by showing a portion of thecharged particle optical column 206, which for example is a column asillustrated in FIG. 3A. The modulation element 24 and the beam stop 226comprise a plurality of charged particle transmitting apertures 46 fortransmitting and/or influencing one or more of the charged particlebeams 8. The cleaning agent 100 is introduced in the space 102 betweenthe modulation element 24 and the beam stop element 226. The cleaningagent is guided towards the beam stop 226, and preferably toward theblanker 24, by conduits 64. Simultaneously, charged particle beams 8 areprojected through the charged particle optical column, at least to thebeam stop 226. The beam stop 226 is provided with a plurality of ventholes 60 enabling a flow path F3 between a first side and a second sideof the beam stop 226. In the illustrated example, contaminants 50 flowfrom the downstream side of the beam stop 226 to the space 102, therebyreducing the pressure at the down-stream side of the beam stop 226. Thespecies passing through the vent hole via path F3, as well as thecleaning agent 100 and contaminants removed from surfaces by thecleaning agent, exit the charged particle optical column as indicated byarrow F4 and flow further toward a vacuum pump connected to the vacuumchamber (not shown). Thereby, the material available for contaminatingthe apertures is reduced. The combination of charged particle beams andcleaning agent has been observed to provide efficient removal ofcontaminants 56, in particular in areas where the charged particle beams8 are present. These areas are often the areas where contaminationlayers are most likely to occur and eventually cause severe disturbanceto the functioning of the charged particle optical elements. This holdsin particularly true for elements comprising small apertures forformation and/or passage of charged particle beams.

If any charged particles would pass through one or more of the ventholes 60, these particles are blocked by non-aperture areas comprised inan element arranged downstream the charged particle optical element, asdescribed with reference to FIG. 3-5.

The cleaning agent, or products thereof, may be prevented from enteringinto the charged particle beam generator module, in particular asdescribed with reference to FIG. 6.

FIG. 10 shows a charged particle system 301 comprising several of thefeatures described above, in particular described with respect to FIGS.3A, 3B, 4 and 6. The charged particle beam system 301 comprises chargedparticle optical elements 24, 226 comprising charged particletransmitting apertures 46. In addition to the charged particletransmitting apertures 46, the charged particle optical element 226 isprovided with vent holes 60. A flow restriction arrangement 74 isprovided between the beam generator module 216 and a second apertureelement 23. The restriction arrangement 74 prevents or at least reducesa flow into the beam generator via the space between the beam generatormodule and the modulation module 225. A cleaning agent source 62 withconduits 64 is provided for directing cleaning agent toward the chargedparticle optical element 226 provided with a plurality of chargedparticle transmitting apertures, and, preferably, also toward thecharged particle optical element 24. A vacuum arrangement 44 isprovided, for maintaining vacuum within the system during operationthereof.

The systems and methods disclosed herein provide not only efficientcleaning within charged particle multi beam systems, but also preventcontamination of apertures within the system. Growth of contaminationlayers is limited by limiting the presence of species formingcontamination layers, as well as by applying cleaning during operationof the system. By removing contamination at a rate higher than the rateat which they accumulate on surfaces, that is, cleaning at overrate,accumulation of contamination is avoided.

The system and method of the present invention have been described byreference to certain embodiments discussed above. These embodiments aresusceptible to various modifications and alternative forms withoutdeparting from the scope of protection defined in the appended claims.

Clauses

-   -   1. Charged particle beam system (201, 301), comprising:        -   a charged particle beam generator (16, 216) for generating a            beam of charged particles (20);        -   a charged particle optical column (206, 306) arranged in a            vacuum chamber,        -   wherein said charged particle optical column is arranged for            projecting said beam of charged particles onto a target            (12), and wherein said charged particle optical column            comprises a charged particle optical element (226) for            influencing said beam of charged particles;        -   a source (62) for providing a cleaning agent (100);        -   a conduit (64) connected to said source and arranged for            introducing said cleaning agent towards said charged            particle optical element;        -   wherein said charged particle optical element comprises:        -   a charged particle transmitting aperture (46) for            transmitting and/or influencing said beam of charged            particles, and        -   a vent hole (60, 60 a, 60 b) for providing a flow path (F3)            between a first side and a second side of said charged            particle optical element,        -   wherein the vent hole has a larger cross section than a            cross section of the charged particle transmitting aperture.    -   2. System according to clause 1, wherein said vent hole has a        cross section of one of the following shapes: circular,        slit-shaped, or elliptical.    -   3. System according to clause 1 or 2, wherein the charged        particle optical element (226) comprises a plurality of said        vent holes (60, 60 a, 60 b) and a plurality of said charged        particle transmitting apertures (46), said vent holes arranged        next to said charged particle transmitting apertures.    -   4. System according to clause 3, wherein said charged particle        transmitting apertures (46) are arranged in one or more groups        and the vent holes are arranged substantially along said one or        more groups.    -   5. System according to clause 4, wherein said vent holes are        arranged in one or more one dimensional arrays.    -   6. System according to clause 4, wherein said vent holes are        arranged in one or more two-dimensional arrays.    -   7. System according to any one of clauses 4 to 6, wherein said        vent holes are arranged on either sides of said one or more        groups of plurality of charged particle transmitting apertures        (46).    -   8. System according to any one of clauses 3 to 7, wherein said        vent holes are arranged immediately adjacent an area comprising        a plurality of said charged particle transmitting apertures        (46).    -   9. System according to any one of clauses 3 to 8, wherein said        vent holes are arranged with a pitch (p) which is equal to or        larger than a dimension of said vent holes, said pitch in        particular being in the range from 1 to 3 times the dimension of        said vent holes.    -   10. System according to clause 9, wherein said pitch is equal to        or larger than a dimension of said vent holes along a direction        of alignment of said vent holes.    -   11. System according to any one of the preceding clauses,        arranged such that a charged particle passing through said vent        hole is prevented from reaching said target.    -   12. System according to any one of the preceding clauses,        wherein said charged particle optical element comprises a beam        stop element (226), said beam stop element comprising:        -   a plurality of charged particle transmitting apertures (46)            for passage of charged particle beams, and a non-aperture            area for blocking passage of charged particles and        -   a plurality of vent holes (60, 60 a, 60 b) for providing a            flow path (F3) through said beam stop element.    -   13. System according to clause 12, said system further        comprising        -   a projection lens (29) comprising a plurality of projection            lens apertures (58) for focusing said charged particle beams            (8), wherein said projection lens is arranged downstream            said beam stop element, and wherein said projection lens and            said beam stop element are arranged such that any charged            particles passing through one or more of said vent holes are            blocked by a non-aperture area of said projection lens.    -   14. System according to clause 13, wherein said vent holes have        a cross section in a range from half of a cross section of said        projection lens apertures to two times the cross section of said        projection lens apertures.    -   15. System according to any one of clauses 13 or 14, wherein        said projection lens further comprises a plurality of dummy        apertures (70) arranged around a group of said projection lens        apertures, wherein said vent holes are arranged such that any        charged particle passing through said vent holes are blocked by        an area located laterally outside said dummy apertures.    -   16. System according to any one of the preceding clauses,        further comprising        -   a second aperture element (23) comprising a plurality of            apertures (66) for forming a plurality of charged particle            beams (8) from said beam (20) of charged particles, said            second aperture element arranged between said charged            particle beam generator and said charged particle optical            element, and        -   a restriction element (76) provided between said charged            particle beam generator and said second aperture element,            said restriction element arranged for preventing or at least            reducing a flow of said cleaning agent and/or products            thereof to said charged particle beam generator.    -   17. System according to clause 16, further comprising:        -   a beam generator module, said charged particle beam            generator being arranged in said beam generator module;        -   a modulation module (225), said second aperture element            being arranged in said modulation module;        -   wherein said restriction element is movably connected to            said beam generator module and arranged for abutting said            modulation module by means of gravity and/or a spring force.    -   18. System according to clause 17, wherein said restriction        element (76) is connected to a first wall (82) of said beam        generator module, said restriction element at least partly        surrounding a perimeter of an opening (80) in said first wall        for passage of said beam of charged particles, wherein said        restriction element comprises an at least partially ring-shaped        element (76), in particular a ceramic ring, said at least        partially ring-shaped element being movably arranged with        respect to said first wall in a direction toward or away from        said modulation module.    -   19. System according to clause 18, further comprising a        confining element (78) for confining a movement of said        restriction element with respect to said first wall.    -   20. System according to clause 19, wherein said restriction        element is provided with one or more protrusions (77) and said        confining element (78) is arranged to cooperate with said        protrusions to confine movement of said restriction element.    -   21. System according to any one of clauses 16-20, further        comprising:        -   a modulation element (24) arranged downstream said second            aperture element (23), said modulation element comprising a            second plurality of apertures (46) for passage of said            charged particle beams and deflectors associated with said            second plurality of apertures, said deflectors arranged to            selectively deflect or not deflect said charged particle            beams, and        -   a beam stop element (226) comprising a third plurality of            apertures (46) for passage of charged particle beams (8) and            a blocking area for blocking charged particle beams, said            beam stop element arranged downstream said modulation            element,        -   said modulation element and said beam stop element arranged            to function together to let pass or to block said            selectively deflected charged particle beams, wherein said            conduit (64) is arranged to direct said cleaning agent            toward said beam stop element and, preferably, also toward            said modulation element.    -   22. System according to any one of the preceding clauses,        wherein electrical connections within said charged particle        optical system are provided with a protective coating, such as        epoxy and/or a metal layer.    -   23. Method for preventing or removing contamination of a charged        particle transmitting aperture (46) in the charged particle beam        system according to any one of the preceding clauses,        -   the method comprising the steps of:            -   introducing a cleaning agent towards said charged                particle optical element while said beam generator (16,                216) is generating said beam of charged particles and/or                while a second charged particle beam source is                generating a beam of charged particles which is directed                toward said charged particle optical element; and            -   maintaining a vacuum in said vacuum chamber (2) while                introducing said cleaning agent,        -   wherein the step of maintaining a vacuum comprises providing            a flow (F3) at least through said charged particle optical            element via said vent hole (60, 60 a, 60 b) to a vacuum pump            connected to said vacuum chamber.    -   24. Method according to clause 23, comprising the step of        preventing any charged particles passing through said at least        one vent hole from reaching said target.    -   25. Method according to clause 23 or 24, wherein said charged        particles passing through said vent hole (60, 60 a, 60 b) are        prevented from reaching said target (12) by blocking these        charged particles by non-aperture areas comprised in a further        aperture element arranged downstream said charged particle        optical element, said further aperture element comprising one or        more apertures for passage of charged particle beams having        passed through said charged particle transmitting apertures.    -   26. Method according to any one of clauses 23 to 25, further        comprising the step of:        -   arranging said charged particle beam system such that a flow            of said cleaning agent or products thereof into said charged            particle beam generator is prevented or at least reduced.    -   27. Method according to any one of clauses 23 to 26, further        comprising the following steps:        -   arranging said charged particle beam generator in a beam            generator module and said charged particle optical element            in a modulation module,        -   providing a restriction element, movably connected to said            beam generator module and abutting said modulation module by            means of gravity and/or spring force.    -   28. Method according to any one of clauses 23 to 27, comprising        introducing said cleaning agent in a region of said charged        particle optical column where said charged particles have energy        in the range of 1-10 kEV, in particular around or lower than 5        keV.    -   29. Method according to any one of clauses 23 to 28, wherein one        or more charged particle beams is present at or near the charged        particle optical element while directing said cleaning agent        toward the charged particle optical element.    -   30. Method for preventing or removing contamination of a charged        particle transmitting aperture in a charged particle beam system        arranged in a vacuum chamber, the charged particle beam system        comprising a charged particle optical column for projecting a        beam of charged particles onto a target, said charged particle        optical column comprising a charged particle optical element for        influencing the beam of charged particles,        -   said charged particle optical element comprises said charged            particle transmitting aperture for transmitting and/or            influencing said beam of charged particles, and at least one            vent hole for providing a flow path from a first side to a            second side of said charged particle optical element;        -   the method comprising the following steps:            -   introducing a cleaning agent towards said charged                particle optical element while a beam of charged                particles is present at or near said charged particle                optical element; and            -   maintaining a vacuum in said vacuum chamber,        -   wherein the step of maintaining a vacuum comprises reducing            a pressure on said first side of said charged particle            optical element by providing a flow through said vent hole,            from said first side to a to a second side of the charged            particle optical element and further to a vacuum pump            connected to said vacuum chamber.    -   31. Method according to clause 30, further comprising one or        more of the features as described in any one or more of clauses        23 to 29.    -   32. A charged particle beam system, comprising:        -   a charged particle beam generator for generating a beam of            charged particles;        -   a charged particle optical column arranged in a vacuum            chamber, wherein the charged particle optical column is            arranged for projecting the beam of charged particles onto a            target, and wherein the charged particle optical column            comprises a charged particle optical element for influencing            the beam of charged particles;        -   a source for providing a cleaning agent;        -   a conduit connected to the source and arranged for            introducing the cleaning agent towards the charged particle            optical element;        -   wherein the charged particle optical element comprises a            charged particle transmitting aperture for transmitting            and/or influencing the beam of charged particles,        -   a second aperture element, comprising a plurality of            apertures for forming a plurality of charged particle beams            from the beam of charged particles, the second aperture            element arranged between the charged particle beam generator            and the charged particle optical element, and        -   a restriction element provided between the charged particle            beam generator and the second aperture element, the            restriction element preventing or at least minimizing a flow            of said cleaning agent and/or products thereof to the            charged particle beam generator.    -   33. System according to clause 30, further comprising one or        more of the features as described in any one or more of clauses        2-22.    -   34. A method for preventing or removing contamination of a        charged particle transmitting aperture in a charged particle        optical element in a charged particle beam system according to        clause 32 or 33, the method comprising the steps of:        -   introducing the cleaning agent towards the charged particle            optical element while the beam generator is generating the            beam of charged particles and/or while a second charged            particle beam source is generating a beam of charged            particles which is directed toward the charged particle            optical element; and        -   maintaining a vacuum in the vacuum chamber while introducing            the cleaning agent,        -   wherein the charged particle beam system is arranged such            that a flow of said cleaning agent or products thereof into            the charged particle beam generator is prevented or at least            minimized.    -   35. Method according to clause 34, further comprising one or        more of the features as described in any one or more of clauses        23-29.    -   36. A charged particle beam system, comprising:        -   a charged particle beam generator for generating a beam of            charged particles;        -   a charged particle optical column arranged in a vacuum            chamber, wherein the charged particle optical column is            arranged for projecting the beam of charged particles onto a            target, and wherein the charged particle optical column            comprises a charged particle optical element for influencing            the beam of charged particles;        -   a source for providing a cleaning agent;        -   a conduit connected to the source and arranged for            introducing the cleaning agent towards the charged particle            optical element;        -   wherein the charged particle optical element comprises a            charged particle transmitting aperture for transmitting            and/or influencing the beam of charged particles, and a vent            hole for providing a flow path between a first side and a            second side of the charged particle optical element,        -   wherein the vent hole are arranged outside an intended            trajectory for the beam of charged particles.    -   37. System according to clause 36, further comprising one or        more of the features as described in any one or more of clauses        1-22.

REFERENCE NUMBER LIST

-   1 multi-beam lithography system-   2 vacuum chamber-   4 charged particle source-   6 charged particle optical column-   7 frame-   8 charged particle beams-   10 target surface-   12 target-   14 optical axes-   16 beam generator module-   18 beam collimating system-   20 collimated electron beam/beam of charged particles-   22 aperture array and condenser lens module-   23 aperture array element/second aperture element-   24 modulation element/beam blanker-   25 modulation module-   26 beam stop element-   287 projection optics module-   29 projection lens-   29 a-c projection lens elements-   30 target support-   32 wafer table-   34 chuck-   36 target support actuator-   38 short stroke actuator-   40 long stroke actuator-   42 lithography control unit-   44 vacuum pump-   46 charged particle transmitting aperture-   48 charged particle optical element-   50 contaminants, residual gases-   52 resist outgassing-   54 adsorption of contaminants-   56 contamination layer-   57 removal of deposits-   58 projection lens aperture-   60 vent hole-   62 source of cleaning agent-   64 conduit-   66 apertures of second aperture element/aperture array-   68 array of apertures in a charged particle optical element-   70 dummy apertures in projection lens-   72 projection of vent holes on projection lens-   74 flow restriction arrangement-   76 restriction element-   77 protrusion-   78 confining element-   80 opening in first wall of beam generator module-   82 first wall of beam generator module-   84 plasma source-   86 plasma chamber-   88 plasma generator coil-   90 precursor gas inlet-   92 plasma chamber outlet-   94 funnel-   96 valve-   98 plasma source controller-   100 cleaning agent-   102 space between blanker and beam stop-   201 charged particle beam system-   206 charged particle optical column-   206 charged particle optical column-   216 beam generator module provided with a flow restricting    arrangement-   225 modulation module-   226 beam stop with vent holes-   228 projection optics module comprising beam stop with vent holes-   301 charged particle beam system-   306 charged particle optical column-   P pitch between vent holes

The invention claimed is:
 1. Charged particle beam system, comprising: acharged particle beam generator for generating a beam of chargedparticles; a charged particle optical column arranged in a vacuumchamber, wherein said charged particle optical column is arranged forprojecting said beam of charged particles onto a target, and whereinsaid charged particle optical column comprises a charged particleoptical element for influencing said beam of charged particles; a sourcefor providing cleaning agent; a conduit connected to said source andarranged for introducing said cleaning agent towards said chargedparticle optical element; a vacuum pump for maintaining a vacuum in saidvacuum chamber; wherein said charged particle optical element comprises:a beam stop element, said beam stop element comprising: a plurality ofcharged particle transmitting apertures for passage of charged particlebeams and a non-aperture area for blocking passage of charged particles,and a plurality of vent holes for providing a flow path through saidbeam stop element, wherein the vent holes have a larger cross sectionthan a cross section of the charged particle transmitting aperture,wherein the system further comprises: a projection lens comprising aplurality of projection lens apertures for focusing said chargedparticle beams, wherein said projection lens is arranged downstream saidbeam stop element, and wherein said projection lens and said beam stopelement are arranged such that any charged particles passing through oneor more of said vent holes are blocked by a non-aperture area of saidprojection lens.
 2. System according to claim 1, wherein said vent holeshave a cross section of one of the following shapes: circular,slit-shaped, or elliptical.
 3. System according to claim 1, wherein saidvent holes are arranged next to said charged particle transmittingapertures.
 4. System according to claim 3, wherein said charged particletransmitting apertures are arranged in one or more groups and the ventholes are arranged substantially along said one or more groups. 5.System according to claim 4, wherein said vent holes are arranged in oneor more one dimensional arrays.
 6. System according to claim 4, whereinsaid vent holes are arranged in one or more two-dimensional arrays. 7.System according to claim 4, wherein said vent holes are arranged oneither sides of said one or more groups of plurality of charged particletransmitting apertures.
 8. System according to claim 3, wherein saidvent holes are arranged immediately adjacent an area comprising aplurality of said charged particle transmitting apertures.
 9. Systemaccording to claim 3, wherein said vent holes are arranged with a pitchwhich is equal to or larger than a dimension of said vent holes, saidpitch in particular being in the range from 1 to 3 times the dimensionof said vent holes.
 10. System according to claim 9, wherein said pitchis equal to or larger than a dimension of said vent holes along adirection of alignment of said vent holes.
 11. System according to claim1, wherein said vent holes have a cross section in a range from half ofa cross section of said projection lens apertures to two times the crosssection of said projection lens apertures.
 12. System according to claim1, wherein said projection lens further comprises a plurality of dummyapertures arranged around a group of said projection lens apertures,wherein said vent holes are arranged such that any charged particlepassing through said vent holes are blocked by an area located laterallyoutside said dummy apertures.
 13. System according to claim 1, furthercomprising a second aperture element comprising a plurality of aperturesfor forming a plurality of charged particle beams from said beam ofcharged particles, said second aperture element arranged between saidcharged particle beam generator and said beam stop element, and arestriction element provided between said charged particle beamgenerator and said second aperture element, said restriction elementarranged for preventing or at least reducing a flow of said cleaningagent and/or products thereof to said charged particle beam generator.14. System according to claim 13, further comprising: a beam generatormodule, said charged particle beam generator being arranged in said beamgenerator module; a modulation module, said second aperture elementbeing arranged in said modulation module; wherein said restrictionelement is movably connected to said beam generator module and arrangedfor abutting said modulation module by means of gravity and/or a springforce.
 15. System according to claim 14, wherein said restrictionelement is connected to a first wall of said beam generator module, saidrestriction element at least partly surrounding a perimeter of anopening in said first wall for passage of said beam of chargedparticles, wherein said restriction element comprises an at leastpartially ring-shaped element, in particular a ceramic ring, said atleast partially ring-shaped element being movably arranged with respectto said first wall in a direction toward or away from said modulationmodule.
 16. System according to claim 15, further comprising a confiningelement for confining a movement of said restriction element withrespect to said first wall.
 17. System according to claim 16, whereinsaid restriction element is provided with one or more protrusions andsaid confining element is arranged to cooperate with said protrusions toconfine movement of said restriction element.
 18. System according toclaim 13, further comprising: a modulation element arranged downstreamsaid second aperture element, said modulation element comprising asecond plurality of apertures for passage of said charged particle beamsand deflectors associated with said second plurality of apertures, saiddeflectors arranged to selectively deflect or not deflect said chargedparticle beams wherein said beam stop element arranged downstream saidmodulation element, said modulation element and said beam stop elementarranged to function together to let pass or to block said selectivelydeflected charged particle beams.
 19. System according claim 1, whereinelectrical connections within said charged particle optical system areprovided with a protective coating.
 20. Method for preventing orremoving contamination of a charged particle transmitting apertures in acharged particle beam system, the charged particle beam systemcomprising: a charged particle beam generator for generating a beam ofcharged particles; a charged particle optical column arranged in avacuum chamber, wherein said charged particle optical column is arrangedfor projecting said beam of charged particles onto a target, and whereinsaid charged particle optical column comprises a charged particleoptical element for influencing said beam of charged particles; whereinsaid charged particle optical element comprises a beam stop element,said beam stop element comprising: plurality of charged particletransmitting apertures for passage of charged particle beams and anon-aperture area for blocking passage of charged particles, and aplurality of vent holes for providing a flow path through said beam stopelement, wherein the vent holes have a larger cross section than a crosssection of the charged particle transmitting aperture, a source forproviding cleaning agent; a conduit connected to said source andarranged for introducing said cleaning agent towards said beam stopelement; a vacuum pump for maintaining a vacuum in said vacuum chamber;a projection lens comprising a plurality of projection lens aperturesfor focusing said charged particle beams, wherein said projection lensis arranged downstream said beam stop element, the method comprising:projecting said beam of charged particles onto said target using saidcharged particle optical column; introducing said cleaning agent fromsaid source via said conduit towards said beam stop element while saidcharged particle beam generator is generating said beam of chargedparticles; and maintaining a vacuum in said vacuum chamber by operatingsaid vacuum pump while introducing said cleaning agent, wherein the stepof maintaining a vacuum comprises providing a flow or movement ofspecies at least through said beam stop element via said vent holes tosaid vacuum pump, and wherein any charged particles passing through saidvent holes are prevented from reaching said target by blocking thesecharged particles by non-aperture areas comprised in said projectionlens, said projection lens comprising a plurality of apertures forpassage of charged particle beams having passed through said chargedparticle transmitting apertures of said beam stop element.
 21. Methodaccording to claim 20, comprising the step of preventing any chargedparticles passing through said vent holes from reaching said target. 22.Method according to claim 20, further comprising the step of: arrangingsaid charged particle beam system such that a flow of said cleaningagent or products thereof into said charged particle beam generator isprevented or at least reduced.
 23. Method according to claim 20, furthercomprising the following steps: arranging said charged particle beamgenerator in a beam generator module and said charged particle opticalelement in a modulation module, providing a restriction element, movablyconnected to said beam generator module and abutting said modulationmodule by means of gravity and/or spring force.
 24. Method according toclaim 20, comprising introducing said cleaning agent in a region of saidcharged particle optical column where said charged particles have energyin the range of 1-10 kEV, in particular around or lower than 5 keV. 25.Method according to claim 20, wherein one or more charged particle beamsis present at or near the beam stop element while directing saidcleaning agent toward the beam stop element.
 26. Method for preventingor removing contamination of a charged particle transmitting aperturesin a charged particle beam system arranged in a vacuum chamber, thecharged particle beam system comprising: a charged particle beamgenerator for generating a beam of charged particles; a charged particleoptical column for projecting the beam of charged particles onto atarget, said charged particle optical column comprising a beam stopelement comprising: a plurality of charged particle transmittingapertures for transmitting and/or influencing said beam of chargedparticles, and a plurality of vent holes providing a flow path from afirst side to a second side of said beam stop element, wherein the ventholes have a larger cross section than a cross section of the chargedparticle transmitting aperture; a vacuum pump for maintaining a vacuumin said vacuum chamber; a source for providing a cleaning agent; aconduit connected to said source and arranged for introducing saidcleaning agent from said source towards said charged particle opticalelement; a projection lens comprising a plurality of projection lensapertures for focusing said charged particle beams, wherein saidprojection lens is arranged downstream said beam stop element, andwherein said projection lens and said beam stop element are arrangedsuch that any charged particles passing through one or more of said ventholes are blocked by a non-aperture area of said projection lens; themethod comprising the following steps: introducing said cleaning agentfrom said source via said conduit towards said beam stop elementcomprising said charged particle transmitting apertures while a beam ofcharged particles generated by said charged particle beam generator ispresent at or near said beam stop element; and maintaining a vacuum insaid vacuum chamber using said vacuum pump, wherein the step ofmaintaining a vacuum comprises providing a flow or movement of speciesat least through said charged particle optical element via said venthole to said vacuum pump.
 27. Method according to claim 20, wherein thecleaning agent comprises one or more of atomic oxygen radicals,molecular oxygen gas, molecular or atomic oxygen ions, and ozone. 28.System according to claim 1, wherein the source for providing a cleaningagent is a plasma source, wherein the plasma source is connected to aframe which supports the charged particle optical column.
 29. Systemaccording to claim 1, further comprising a modulation element arrangedupstream the beam stop element, said modulation element comprising asecond plurality of apertures for passage of said charged particle beamsand deflectors associated with said second plurality of apertures, saiddeflectors arranged to selectively deflect or not deflect said chargedparticle beams, wherein said conduit is arranged for directing thecleaning agent also to the modulation element or wherein said systemfurther comprises a second conduit arranged for directing the cleaningagent to the modulation element.